Aircraft are typically designed to contain fuel in their wings. This fuel is pumped between tanks and to the engines by relatively low power pumps at a rate capable of sustaining the maximum fuel usage of the engines, with a margin of approximately 50%.
Aerial refueling tanker aircraft additionally typically have much more powerful fuel pumps designed to ensure that fuel is dispensed to a receiving aircraft within a reasonably short time. Tanker aircraft typically store fuel in multiple tanks from which fuel can either be used for normal aircraft operation or for refueling following aircraft, e.g. via a trailing hose dispense station. To achieve fast dispense rates it is desirable to pump fuel from several tanks simultaneously to one or more of a plurality of dispense stations.
A conventional aerial refueling system, such as that described in US 2006/0278761, has a plurality of fuel pumps of a specified power rating and which are each selectively energized during fuel transfer and refueling operations. The fuel pumps are connected via a common gallery to a plurality of dispense stations.
It is generally desirable to achieve a high rate at which fuel can be dispensed from a tanker aircraft to following aircraft. However, simply up-rating the power of existing fuel pump technology presents challenges in the design of other aircraft systems.
If a fuel pipe ruptures in a tank then an inadvertent transfer will take place into that tank. Typically the fuel will then fill that tank and cause its pressure to increase.
Normally when a tank hits the overflow level an alert is created to the crew who will resolve the situation by shutting down the offending pump(s). The overpressure caused by this type of event is adequately catered for by the design strength of the wing and the available overflow paths. An attempt to increase the fuel dispense rate using existing fuel pump technology may necessitate strengthening of the wing structure and/or the overflow system, which could add unacceptable weight to the aircraft.
A pump capable of dispensing fuel at a high rate typically has a high pressure outlet when the pump is turned on just prior to commencing an aerial refueling operation. When fuel starts flowing to the receiving aircraft the pressure then drops significantly. On disconnect of the receiving aircraft the pressure then steps back again to the maximum until the pump is switched off. These step changes in pressure can cause “pipe-hammer” which can contribute to pipe or connector rupture. An attempt to increase the fuel dispense rate using existing fuel pump technology may exacerbate these step changes in pressure and could increase the risk of a pipe rupture event.
If fuel is being dispensed from multiple tanks simultaneously then the tank nearest to the active dispense station will typically deplete faster than those further along the fuel gallery due to flow resistance which is dependent on the distance the fuel travels through the network of pipes and valves. Also, fuel pumps typically have a wide variation in performance, perhaps 5-10%, hence it is not possible to rely on matching the pumps for equal flows. At high dispense rates this will affect the lateral and/or longitudinal weight balance of the aircraft due to the difference between the rate of depletion of the fuel volume in the various tanks. If an unbalanced fuel load follows a dispense operation then the crew may need to perform manual cross-feeds before the next dispense, or to rebalance for the return to base flight sector. Unbalanced wing fuel loads result in intervention from the flight control system which compensates the imbalance with flap settings, resulting in less efficient flight. An attempt to increase the fuel dispense rate using existing fuel pump technology may increase the level of crew intervention required to rebalance the aircraft, and may even extend beyond the crew's capabilities within the aircraft operational envelope.